1. Field of the Invention
The invention relates in general to insulated transmission lines and in particular to compressed gas insulated transmission lines having reduced electric field regions.
2. Description of the Prior Art
In general two common types of insulators are used for supporting the inner high voltage conductor centrally within the outer conductor of a compressed gas insulated transmission line. A post insulator having metallic inserts at both ends wherein the post simply screws onto a stud secured in the inner conductor and an insulator which is cast in place around the inner conductor onto a thin metal sleeve which is secured to the inner conductor. The post insulator is relatively low in cost and has a high flashover voltage. The major disadvantage is that as a result of the metallic inserts, although the external surface stress is low, the internal stress of certain post insulators is high, which may result in volume punctures through the post insulator. The high internal stress is due to the fact that the voltage gradient between the inner and outer conductor must be withstood by that portion of the insulator which lies between the metallic inserts and therefore is subjected to an increased electric field greater than the field present in the surrounding gas insulating medium. Insulators which are cast around the inner conductor are not subjected to this phenomenon since internal fields are lower. However, both types of insulator are subjected to high electrical fields at the critical insulator-conductor interface region, which can lead to flashover and a reduced life expectancy for the insulator. Accordingly, it would be desirable to have a transmission line wherein portions of the inner conductors are arranged to produce regions of reduced electrical field at the insulators. Further, it would be desirable if the arrangement or structure of the portion of the inner conductor that decreased the electric field would also provide for flexibility of the inner conductor, which is desirable for the design of flexible transmission lines.